During the harvesting of forage material it is common practice to drive a truck or wagon alongside of the harvester for the purpose of periodically filling the truck with materials that have been accumulated by the harvester during the truck's absence to discharge its load. It is desirable for the harvester to have a large transporting box associated with it so that the harvester may be operated, and forage materials accumulated within the harvester, while the truck is absent. High capacity, self-unloading forage hoppers have been disclosed in U.S. Pat. application Ser. Nos. 06/354,938 and 06/354,906, owned by the assignee of the present invention, that maximize the space of the container box while enabling the harvester to meet governmental height and width restrictions with respect to vehicles which at least periodically utilize public roads. These previously disclosed harvesters have included a tiltable container box and a retractable elevator for emptying the contents of the container box. It would be desirable, however, to provide a tiltable crop container box having a retractable elevator with an elevator control mechanism that would automatically position the elevator to its fully extended position simultaneously with the tilting of the container box to its operating position. Moreover, it would be desirable for such a position control mechanism to permit positioning of the elevator to its fully extended position independently of the position of the container box so that crop material could be discharged from the elevator mechanism at different heights to accommodate unloading of crop material to different sized trucks without regard to the position of the container.